Axial flux permanent magnet generator/motor

ABSTRACT

The present invention is related to a permanent magnet electrical generator/motor comprising a stator, a rotor, a rotational axis and an air gap between stator and rotor so that the magnetic flux across said air gap is essentially oriented along said rotational axis, said rotor preferably comprising a plurality of permanent magnets to form rotor poles, said generator/motor being characterised in that said stator comprises a disc ( 1 ), and a plurality of magnetic cores ( 2 ), said cores ( 2 ) being attached to said disc ( 1 ) by attachment means, said cores comprising coils ( 6 ) wound round the legs of said cores. Furthermore, said machine comprises means for distributing the flux across the air gap between rotor and stator. Preferably, this takes on the form of flux distribution plates ( 28,29 ), placed between rotor and stator.

FIELD OF THE INVENTION

The present invention is related to an axial flux permanent magnetgenerator/motor, for example for use with internal combustion engines,where this type of machine is incorporated in the flywheel.

STATE OF THE ART

The axial flux permanent magnet generator and motor has been describedin various forms. It consists of a stator comprising a plurality ofcoils, and a rotor comprising several permanent magnets, wherein an airgap is present between stator and rotor. The notion of ‘axial flux’ isnot always defined in the same way. In this text, the expression ‘axialflux’ is related to machines wherein the flux across the air gap isessentially parallel to the machine's rotational axis.

The closest prior art when it comes to this type of machine has beendisclosed in the publication entitled ‘An Axial Flux Interior PMSynchronous Machine’, by D. H. Kang et al., ICEM 2000, 28-30 Aug. 2000.It comprises two stators, each made as a single hollow cylinder withradial slots cut out of one of the ring-shaped surfaces. The coils arethen wound round the radial extensions left in between the removedslots. The manufacture of such a stator is complex as well as expensive,especially in small series.

AIMS OF THE INVENTION

The present invention aims to provide a new axial flux permanent magnetgenerator/motor, with reduced complexity and cost compared to the priorart, whilst presenting solutions for problems intrinsic in thesimplified design according to the invention.

SUMMARY OF THE INVENTION

The present invention is related to a permanent magnet electricalgenerator/motor comprising a stator, a rotor, a rotational axis and anair gap between stator and rotor so that the magnetic flux across saidair gap is essentially oriented along said rotational axis, said rotorcomprising a plurality of locations for placing permanent magnets orelements of magnetic material to form rotor poles, said generator/motorcomprising a first non-magnetic disc and a plurality of magnetic cores,said cores being attached to said first disc by attachment means, saidcores comprising coils, said generator/motor being characterized in thatit further comprises a number of flux distribution plates in between thestator and the rotor, said plates being made of magnetic material.

These flux distribution plates may be in the form of a separate plateplaced before each pole. Alternatively, these flux distribution platesmay be designed as an inner and an outer flux distribution plate, madeof magnetic material, said plates being present in between the statorand the rotor, said plates comprising radial extensions, the extensionsof the inner plate pointing radially outward with respect to themachine's rotational axis, and the extensions of the outer platepointing radially inward, so that each extension of said inner plate isadjacent an extension of said outer plate, with an air gap in betweeneach pair of said adjacent extensions. The edges of said air gap, whichare formed by the edges of two adjacent extensions are preferablyparallel to each other.

The flux distribution plates may be attached to the rotor, so that eachof said extensions covers at least the area of one permanent magnetlocation.

Alternatively, the flux distribution plates may be attached to thestator, so that each of said extensions covers at least the area of thecross section of one leg of a core.

According to one embodiment, the contour of said extensions comprisestwo straight portions, and a circular portion connecting said twostraight portions, so that two adjacent straight portions of twoadjacent extensions are parallel to each other and at an angle to anyradius of said rotor which is crossing said two adjacent straightportions.

Flux distribution plates according to the invention may compriseadjacent strips of magnetic material. According to the preferredembodiment, said flux distribution plates comprise cuts through thecomplete thickness of the plates, in those parts of the plates whichface the rotor poles.

The invention is equally related to a permanent magnet electricalgenerator/motor comprising a stator, a rotor, a rotational axis and anair gap between stator and rotor so that the magnetic flux across saidair gap is essentially oriented along said rotational axis, said rotorcomprising a plurality of locations for placing permanent magnets orelements of magnetic material to form rotor poles, said generator/motorcomprising a first non-magnetic disc and a plurality of magnetic cores,said cores being attached to said first disc by attachment means, saidcores comprising coils, said generator/motor being characterized in thatthe rotor comprises locations which are shaped so that their contourparallel to the rotor surface comprises two concentric circular portionsand two connecting portions connecting the end points of said circularportions, wherein two adjacent connecting portions of two adjacentmagnets are parallel to each other.

Said means for attaching said cores to said first disc may compriseL-profiles and bolts.

A generator/motor according to the invention may further comprise a ringof magnetic material, connected to the rotor on the side opposite thestator. This ring may also be made of laminated magnetic material.

According to one embodiment, the generator/motor according to theinvention further comprises a second non-magnetic disc, said cores beingpresent between said first and second disc. According to anotherembodiment, said coils are attached to said cores by glueing.

A generator/motor according to the invention may further comprise asecond stator, which is placed opposite the first stator on the otherside of said rotor. Said second stator may be rotated over an anglearound the rotational axis of said generator/motor, with respect to saidfirst stator.

A generator/motor according to the invention may further comprise atleast one electronic converter, which works in “boost-mode” at low speedand as a rectifier at high speed.

The rotor may comprise in every one of said locations a permanentmagnet. Alternatively, the rotor may comprise a number of permanentmagnets and a number of elements made of magnetic material.

In another design according to the invention, the rotor may compriseonly elements made of magnetic material and placed into said permanentmagnet locations, in such a way that the number of rotor poles isdifferent from the number of stator poles.

The cores which are used on the stator may be U-shaped cores or E-shapedcores.

In the case of 3 or more phase machine, the generator/motor according tothe invention preferably further comprises a ring made of laminatedmagnetic material, said ring connecting the cores of said stator.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a photograph of a partly assembled stator according to theinvention.

FIG. 2 shows a photograph of a complete rotor according to theinvention.

FIGS. 3 a and 3 b represent overviews of an axial flux generator/motoraccording to the invention.

FIG. 4 represents a first sectional view of the stator of the invention,as shown in FIG. 3 a.

FIG. 5 represents a second sectional view of the stator of theinvention, as shown in FIG. 3 a.

FIG. 6 represents a sectional view of the rotor of the invention, asshown in FIG. 3.

FIG. 7 represents a view of a rotor according to a second embodiment ofthe invention.

FIG. 8 represents a view of a pair of flux distribution plates, whichcomprise cuts.

FIG. 9 represents a view of a ring of laminated magnetic material, whichcan be installed on the stator in case of a 3 or more phase machine.

FIG. 10 illustrates the rotation of one stator with respect to theother, in the embodiment of the invention comprising two stators.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally presents a concept for an axial fluxpermanent magnet generator/motor, wherein a number of cores 2 areattached onto a disc 1, in order to form the stator. The disc 1 ispreferably circular shaped, but the function of this disc can beperformed by any plate made of a non-magnetic material. Also, the discor plate 1 may be made of a magnetic material, which is covered by aninsulation. The photograph of FIG. 1 illustrates a partly assembledstator according to the invention, comprising six U-cores. The fact thatseparate cores are used allows the use of standard cores, thus leadingto a straightforward and inexpensive design.

The main problem of using such a design however, resides in the largefluctuation of the air gap between such a stator and the rotor, which isto be placed facing the cores on the stator. The rotor will preferablycomprise a number of permanent magnets of a given form, at equaldistances on a circle which is co-axial with respect to the machine'srotational axis. The large air gap fluctuation is then being caused bythe large open spaces left in-between adjacent cores 2 on the disc 1, aswell as in between the legs of one core 2, making the changeover betweentwo adjacent poles difficult. The present invention solves this problemby providing an extra feature in the form of means for distributing theflux in an optimal way across the whole of the air gap's cross sectionperpendicular to the machine's rotational axis.

In order to do this, and according to the preferred embodiment, therotor or the stator are equipped with so-called ‘flux-distributionplates’, which are specially shaped plates made of magnetic material,placed in the path of the axial flux between rotor and stator, and whichallow the change-over fluctuation to be reduced to a minimum. A possibleshape of these plates 28, 29 when attached to the rotor, are illustratedin FIG. 2.

According to a further embodiment, the feature of flux distribution isobtained by shaping the permanent magnets themselves in a pre-definedway.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

A preferred embodiment of the machine of the invention is shown indetail in FIG. 3 a. It concerns a mono-phase disc permanent magnetgenerator, equipped with twelve cylinder-shaped permanent magnets 25 andflux-distribution plates 28,29 on the rotor. Sectional views indicatedin FIG. 3 are shown in subsequent FIGS. 4 to 6. The stator comprises adisc 1, made of non-magnetic material, onto which six magnetic cores 2are attached. These are U-shaped cores and can be any type of standardcore, made of laminated magnetic material. They are attached to the disc1 by a number of L-shaped profiles 3, bolted onto the disc 1 by bolts 4and onto the cores 2 by bolts 5. The cores 2 are attached in such a waythat the upstanding legs are essentially perpendicular to the disc 1.

The coils 6 are preferably wound round coil formers 7, which are thenplaced on the upstanding legs of the cores 2. A second non-magnetic disc8 is preferably added opposite the first disc 1, so that both discs takecare of the clamping of the cores with sufficient strength and of thejamming of the coils in the cores. Rectangular openings are present inthe second disc 8 corresponding to the top surfaces 9 of the legs of theU-shaped cores.

The first and second disc (1,8) are held together by a set of bolts 20,in combination with tubes 21. This assembly is then attached through anadditional set of bolts 23 and tubes 24 to a stationary plate 22, whichrepresents the structure onto which the machine is installed.

The rotor is positioned in between this stationary plate 22 and thesecond disc 8. This rotor comprises a number of permanent magnets 25with alternated orientation of north and south poles. The permanentmagnets are cylinder-shaped. Standard magnets of this type may be used,such as loudspeaker magnets. The magnets are clamped into a non-magneticcircular disc 26.

At the back of this non-magnetic disc 26, the permanent magnets areconnected by a ring 27 of magnetic material, in order to create amagnetic path. On the surface directly facing the stator, the rotor isequipped with two flux distribution plates 28 and 29. These are bestillustrated in FIG. 6 and on the photograph of FIG. 2. In FIG. 6, themagnets 25 are made visible, for clarity's sake. The flux distributionplates are made of magnetic material. Each plate is essentially aring-shaped disc with radial extensions covering one type (north orsouth) of magnetic pole. The inner plate 28 is attached by bolts 30 tothe rotor disc 26 and comprises radial extensions 31, which are pointingoutward from the central ring into which the bolts 30 are present. Eachof these extensions 31 covers the complete area of one type of permanentmagnet. The outer plate 29 is equally bolted by bolts 32 to the rotordisc 26, and comprises radial extensions 33 which are pointing inwardfrom the circumferential ring where the bolts 32 are present. Theextensions 33 are covering the area of the remaining magnets, in betweenthe magnets which are already covered by the inner plate 28. The surfaceof the extensions 31,33 is preferably larger than the surface of themagnets which they cover.

The specific contour of the extensions in this preferred embodiment canbe seen to comprise two straight portions 40 and a circular portion 41,essentially concentric with the rotor. The straight portions 40 are notoriented along a radius of the rotor, but are at an angle to any rotorradius crossing said straight portions. As a consequence, the air gap 34between two adjacent extensions is at an angle to the rotor radius. Thereason for this is explained after this.

Every extension 31 of the inner plate 28 is adjacent an extension 33 ofthe outer plate 29. The air gap 34 between these flux distributionplates is larger than the air gap between the flux distribution platesand the magnetic material of the stator, in order to avoid flux linescrossing the air gap between flux distribution plates.

The function of these flux distribution plates is to mitigate the effectof the large fluctuation in air gap volume, as a consequence of the useof separate U-cores on the stator, which might otherwise lead to theproblem of torque pulsation (cogging). For this purpose, the area of theextensions 31,33 is as large as possible, with the limitation that theair gap 34 between adjacent extensions needs to be larger than the airgap between rotor and stator. Furthermore, the air gap 34 between theflux distribution plates 28 and 29 is designed at an angle to the radiusof the rotor, as a further measure against torque pulsation. This way,the changeover between two poles with different orientation can takeplace with a minimal variation in air gap volume between the fluxdistribution plates and the magnetic material of the stator.

The rotor disc is attached to a central part 35, which is to beconnected to the machine axis. The connection between this central partand the rotor disc is supplied by bolts 36.

ALTERNATIVE EMBODIMENTS OF THE INVENTION

FIG. 3 b shows a slightly modified embodiment which is essentiallyequivalent to the one shown in FIG. 3 a, and wherein the second disc 8has been omitted, along with the bolts 20 and tubes 21. In thisembodiment, the clamping of the coils onto the cores is done by otherattaching means, preferably by glueing the coils 6 to the cores 2.

The flux distribution plates may be attached to the stator, facing therotor, instead of to the rotor. The same form of plates may be used inthis case. The effect obtained by these plates, put onto the stator isexactly the same as in the above described case. Each extension 31 or 33of one of both plates then covers at least the area of the cross-sectionof one leg of a core 2.

The form of flux distribution plates is not limited to the one shown inthe figures. The main characteristic of such a plate is that itessentially covers at least the area of one rotor or stator pole. It isalso possible to provide a separate flux distribution plate for everyrotor pole.

The magnet arrangement is not necessarily based on a series ofalternating north and south-oriented permanent magnets, but may also bebased on groups of north-oriented magnets adjacent to groups ofsouth-oriented magnets. This is the preferred case for example in a 3 ormore phase machine according to the invention. One rotor pole thencomprises a group of similarly oriented magnets. In this case therefore,flux distribution plates installed on the rotor may have fewer butlarger extensions 31,33 which cover such a group of adjacent magnets ofthe same orientation. Alternatively, separate flux distribution platesmay be installed, each covering one rotor pole.

The flux may also be distributed in the same way by shaping thepermanent magnets themselves in a given way. This effect is obtained byusing magnets which have the shape of the radial extensions (31,33) ofthe flux distribution plates 28 and 29. A rotor of this kind, equippedwith specially shaped magnets 45 is shown in FIG. 7. These magnets arethen equally clamped into a non-magnetic disc 26. No flux distributionplates are required in this embodiment. The magnets of FIG. 7 have twoconcentric circular portions 50 and two connecting portions 51 betweenthe end points of said circular portions. In this case, the connectingportions 51 are straight lines, which are not parallel to the rotorradius, in order to avoid torque pulsation.

For both the latter embodiment, as the one with flux distributionplates, it is not required that the adjacent edges of the extensions31,33 or of the magnets 45 are straight lines. These edges may have anyshape. It is however preferred that adjacent edges are parallel.

In order to adapt it to a 3 or more phase machine, the rotor of FIG. 7may also comprise a smaller number of larger magnets 45, equallycomprising portions 50, 51 but each taking up a larger space in therotor's circumference.

Instead of U-shaped cores, E-shaped cores may be used.

In the machine shown in FIG. 3, it is not necessary to use permanentmagnets in every position 25. It is also possible to have a rotorwherein every second position is filled by a permanent magnet. This maybe a solid permanent magnet or a permanent magnet made of laminatedmaterial. For example, all magnets having their north pole at the leftin FIG. 3 may be replaced by a disc made of magnetic material,preferably laminated magnetic material. Because it is not required tohave permanent magnets at every position 25 around the rotor, as shownin FIG. 6, these positions may more generally be called ‘locations’, tobe filled in either by permanent magnets or_by elements of laminatedmagnetic material of the same or other shape as the permanent magnets.

The ring 27 at the back of the rotor is preferably formed of a laminatedmaterial.

To limit the magnetic losses in the flux distribution plates 28,29 theseplates or at least part of these plates preferably comprise‘interruptions’ perpendicular to the plate's plane surface. In general,the plate may be said to comprise a series of adjacent strips ofmagnetic material, lying next to each other, and thereby making up theplate or a part of the plate.

In the preferred embodiment, the plates are machined, for example by alaser tool, in order to form cuts 37 which go through the completethickness of the plates and which are located at least in those parts ofthe plate facing the rotor poles, see FIG. 8. Parts on either side ofsuch a cut 37 are understood to be the ‘adjacent strips’ in the moregeneral description.

In a special design for the 3 or more phase machines the magnetic coresof the stator can be magnetically combined by layered rings of magneticmaterial. FIG. 9 shows such a layered ring 52, comprising open spaces 53which are to be placed over the cores 2 on the stator. The ring 52 isthen placed in between the disc 1 and the coil formers 7.

For the 3 or more phase system, a special rotor construction can be madeso that the machine works as a switched reluctance motor or generatorwithout the use of permanent magnets on the rotor. All of the rotorpoles can then be built by elements of magnetic material, preferablylaminated, with a special design resembling the radial extensions 31, 33in order to produce the same effect as with the flux distributionplates, or by preferably laminated magnetic elements, equipped with fluxdistribution plates as described above. For the switched reluctancemachine the pole number of the rotor differs from the pole number of thestator. The equilibrium position of the rotor (minimal reluctance) turnswith the alternate excitation of the different stator poles.

It is also possible to make a twin type of machine by replacing thestator construction at the front of the rotor by two similar statorconstructions one at each side of the rotor. For this design, the rotordoes not need the magnetic ring 27 at the backside since the magneticpath is already closed at each side of the rotor by the magnetic coresof the stator. The flux distribution plates 28,29 however or the specialdesign of the permanent magnets have to be present at both sides of therotor for this construction. According to the preferred embodiment, thestators are rotated over an angle with respect to each other, around therotational axis, as is illustrated in FIG. 10. This is preferred inorder to reduce torque pulsations.

The machine can only generate a controlled output voltage if anelectronic converter is being used. The converter works in “boost-mode”at low speed and as a rectifier at high speed. For the 3-phaseconstruction, the electronic converter has the additional function ofmotor control when the machine is used as a motor.

It is possible to generate a 3-phase output with the 1-phaseconstruction by using 3 electronic converters each on a 1-phase output.

To reduce the weight of the machine, it is possible to make the coils 6of the stator with aluminium foil instead of with copper wire.

1. A permanent magnet electrical generator/motor comprising a stator, arotor, a rotational axis and an air gap between stator and rotor so thatthe magnetic flux across said air gap is essentially oriented along saidrotational axis, said rotor comprising a plurality of locations forplacing permanent magnets (25) or elements of magnetic material to formrotor poles, said generator/motor comprising a first non-magnetic disc(1), and a plurality of magnetic cores (2), said cores (2) beingattached to said first disc (1) by attachment means, said corescomprising coils (6), said generator/motor being characterized in thatsaid generator/motor further comprises a number of flux distributionplates in between the stator and the rotor, said plates being made ofmagnetic material.
 2. A generator/motor according to claim 1, comprisingan inner (28) and an outer (29) flux distribution plate, made ofmagnetic material, said plates comprising radial extensions (31,33), theextensions (31) of the inner plate (28) pointing radially outward withrespect to the machine's rotational axis, and the extensions (33) of theouter plate (29) pointing radially inward, so that each extension (31)of said inner plate is adjacent an extension (33) of said outer plate,with an air gap (34) in between each pair of said adjacent extensions(31,33).
 3. A generator/motor according to claim 2, wherein the edges ofsaid air gap (34), which are formed by the edges of two adjacentextensions (31,33) are parallel to each other.
 4. A generator motoraccording to claim 2, wherein said flux distribution plates (28,29) areattached to the rotor, and wherein each of said extensions (31,33)covers at least the area of one permanent magnet location.
 5. Agenerator/motor according to claim 2, wherein said cores (2) comprise anumber of upstanding legs around which said coils (6) are wound, andwherein said flux distribution plates are attached to the stator,wherein each of said extensions (31,33) covers at least the area of thecross section of one leg of a core (2).
 6. A generator/motor accordingto claim 2, wherein the contour of said extensions (31,33) comprises twostraight portions (40), and a circular portion (41) connecting said twostraight portions, so that two adjacent straight portions of twoadjacent extensions (31,33) are parallel to each other and at an angleto any radius of said rotor which is crossing said two adjacent straightportions (40).
 7. A generator/motor according to claim 1, wherein saidflux distribution plates comprise adjacent strips of magnetic material.8. A generator/rotor according to claim 7, wherein said fluxdistribution plates (28,29) comprise cuts (37) through the completethickness of the plates, in those parts of the plates which face therotor poles.
 9. A generator/motor according to claim 1, wherein aseparate flux distribution plate is provided for every rotor pole.
 10. Apermanent magnet electrical generator/motor comprising a stator, arotor, a rotational axis and an air gap between stator and rotor so thatthe magnetic flux across said air gap is essentially oriented along saidrotational axis, said rotor comprising a plurality of locations forplacing permanent magnets (25) or elements of magnetic material to formrotor poles, said generator/comprising a first non-magnetic disc (1),and a plurality of magnetic cores (2), said cores (2) being attached tosaid first disc (1) by attachment means, said cores comprising coils(6), characterized in that said locations are shaped so that theircontour parallel to the rotor surface comprises two concentric circularportions (50) and two connecting portions (51) connecting the end pointsof said circular portions (50), wherein two adjacent connecting portions(51) of two adjacent locations are parallel to each other.
 11. Agenerator/motor according to claim 1, wherein said means for attachingsaid cores (2) to said first disc (1) comprise L-profiles (3) and bolts(4,5).
 12. A generator/motor according to claim 1, further comprising aring (27) of magnetic material, connected to the rotor on the sideopposite the stator.
 13. A generator/motor according to claim 1, furthercomprising a second non-magnetic disc (8), said cores (2) being presentbetween said first and second disc.
 14. A generator/motor according toclaim 1, wherein said coils (6) are attached to said cores by glueing.15. A generator/motor according to claim 1, further comprising a secondstator, which is placed opposite the first stator on the other side ofsaid rotor,
 16. A generator/motor according to claim 15, wherein saidsecond stator is rotated over an angle around the rotational axis ofsaid generator/motor, with respect to said first stator.
 17. Agenerator/motor according to claim 1, further comprising at least oneelectronic converter, which works in “boost-mode” at low speed and as arectifier at high speed.
 18. A generator/motor according to claim 1,wherein the rotor comprises in every one of said locations, a permanentmagnet.
 19. A generator/motor according to claim 1, wherein the rotorcomprises in said locations a number of permanent magnets and a numberof elements made of magnetic material.
 20. A generator/motor accordingto claim 1, wherein the rotor comprises only elements made of magneticmaterial and placed into said permanent magnet locations, in such a waythat the number of rotor poles is different from the number of statorpoles.
 21. A generator/motor according to claim 1, wherein said cores(2) are U-shaped cores or E-shaped cores.
 22. A generator/motoraccording to claim 1, further comprising a ring (52) made of laminatedmagnetic material, said ring connecting the cores (2) of said stator.